1. Field of the Invention
The present invention relates to voltage regulator circuits. More particularly, the invention relates to the sensing of output current delivered to a load by a buck-type DC-to-DC switched mode power converter.
2. Description of Related Art
Switched mode DC-to-DC power converters are commonly used in the electronics industry to convert an available direct current (DC) level voltage to another DC level voltage. A switched mode converter provides a regulated DC output voltage by selectively storing energy by switching the flow of current into an output inductor coupled to a load. A synchronous buck converter is a particular type of switched mode converter that uses two power switches, such as MOSFET transistors, to control the flow of current in the output inductor. A high-side switch selectively couples the inductor to a positive power supply while a low-side switch selectively couples the inductor to ground. A pulse width modulation (PWM) control circuit is used to control the gating of the high-side and low-side switches. Synchronous buck converters generally offer high efficiency and high power density, particularly when MOSFET devices are used due to their relatively low on-resistance. Therefore, synchronous buck converters are advantageous for use in providing power to electronic systems having demanding power requirements, such as microprocessors that require a control voltage (Vcc) of 1 to 1.5 volts with current ranging from 40 to 60 amps. For certain applications having especially high current load requirements, it is known to combine plural synchronous buck converters together in multi-phase configurations operated in an interleaf mode.
To regulate the performance of a synchronous buck converter, it is known to monitor the amount of current sent to the load. This information is important to protect the load from damage caused by excessive current, to ensure that sufficient current is delivered to the load in view of changing load conditions (i.e., controlling voltage xe2x80x9cdroopxe2x80x9d caused by a step up in load), and to permit current sharing between phases of multi phase configurations. One approach to measuring the load current is to include a sensing resistor in series with the output inductor and to monitor the voltage drop across the sensing resistor. The sensing resistor must have a resistance value large enough to keep the sensed voltage signal above the noise floor, as the voltage drop can be measured more accurately with a higher resistance value. A significant drawback of this approach is that the sensing resistor wastes the output energy and thereby reduces the efficiency of the synchronous buck converter. Moreover, the sensing resistor generates heat that must be removed from the system.
Another approach to measuring the load current is to place the sensing resistor in series with the drain of the high-side switch (i.e., MOSFET) and monitor the voltage drop across the sensing resistor as in the preceding approach. In this position, the amount of energy dissipated by the sensing resistor is substantially less than in the aforementioned position in series with the output inductor. A drawback of this approach is that the high-side switch changes state at a relatively high rate (e.g., greater than 250 KHz) and, as a result, the high-side switch current is discontinuous. When the high-side switch turns on, the current through the switch and the sensing resistor starts at zero and increases rapidly before settling and then returning to zero when the high-side switch turns off. The information obtained from sampling the voltage across the sensing resistor must therefore be utilized during a subsequent switching cycle, making it necessary to include xe2x80x9csample and holdxe2x80x9d circuitry to store the sampled information from cycle to cycle. Not only does this add complexity to the converter, but there is also a time delay in regulating the output current that diminishes the stability of the converter.
In yet another approach to measuring the load current, a current sensor is included in parallel with the output inductor. The current sensor includes a resistor and capacitor connected together in series. The signal passing through the output inductor has a DC component and an AC component. The output inductor is comprised of a wire, such as copper, that has an inherent resistance per unit length that results in a DC resistance value. The AC component of the signal depends on the inductance and internal resistance values of the output inductor, as well as the resistance and capacitance of the current sensor. By selecting the values of the resistor and capacitor to define a time constant having a known relationship with the corresponding time constant of the output inductor, the instantaneous voltage across the capacitor can be made equal to the voltage across the DC resistance of the inductor and thereby proportional to the instantaneous current through the output inductor. Thus, the output inductor current can be sensed without dissipating the output energy by monitoring the voltage across the capacitor. A drawback of this approach is that the DC resistance of the output inductor will change with temperature since the copper material of the output inductor has a thermal coefficient. This introduces a DC error that affects the accuracy of the measurement of the output inductor current and diminishes the stability of the converter.
Accordingly, it would be desirable to provide a way to accurately sense the output current delivered to a load by a buck-type DC-to-DC switched mode power converter that corrects for thermal variation.
The present invention provides an apparatus and method for accurately sensing the output current delivered to a load by a buck-type DC-to-DC switched mode power converter that corrects for thermal variation. According to the invention, a first current sense signal provides a fast indication of output current of the DC-to-DC converter that is susceptible to thermal variation of the output inductor of the converter, and a second current sense signal provides a slow but accurate indication of output current that is not affected by said thermal variation. The first current sense signal is corrected using the second current sense signal to yield accurate output current sensing information.
In an embodiment of the invention, an output current sensing apparatus is provided for use in a multi-phase DC-to-DC voltage converter comprising a plurality of converter modules connected to a common load and having a common input voltage source. A first current sensor is adapted to be coupled to an output inductor of at least one of the plurality of converter modules to derive a first current sense signal corresponding to current passing through an internal DC resistance of the output inductor. A second current sensor is adapted to be coupled to a sensing resistor disposed in series between the common input voltage source and the load to derive a second current sense signal corresponding to current passing through the sensing resistor. A current sense circuit receives the first and second current sense signals. The current sense circuit filters the second current sense signal, integrates a difference between the first current sense signal and the filtered second current sense signal, and adjusts the first current sense signal based on the integrated difference. The current sense circuit thereby provides the first current sense signal as a slope signal for controlling a conduction duty cycle of the DC-to-DC converter and a DC level signal corresponding to a thermal drift error of the slope signal.
More particularly, the first current sensor further comprises a resistor and a capacitor connected together in series and connected in parallel with the output inductor of the at least one of the plurality of converter modules. With regard to the second current sensor, the sensing resistor is further disposed in series between the common input voltage source and a high-side conduction switch of the plurality of converter modules. The second current sensor further comprises a differential amplifier adapted to measure a voltage across the sensing resistor and a sample and hold circuit that stores the voltage for at least one conduction cycle of the DC-to-DC converter. Alternatively, the sensing resistor may be disposed in series between the output inductor of the at least one of the plurality of converter modules and the load. In that case, the second current sensor further comprises a differential amplifier adapted to measure a voltage across the sensing resistor, and a sample and hold circuit is not needed.
In another embodiment of the invention, a method for sensing output current in a multi-phase DC-to-DC voltage converter is provided. A first current sense signal is derived corresponding to current passing through an internal DC resistance of an output inductor of at least one of the plurality of converter modules. The first current sense signal provides a relatively fast indication of output current of the multi-phase DC-to-DC converter that is susceptible to thermal variation of the at least one output inductor. A second current sense signal is derived corresponding to current passing through a sensing resistor disposed in series between the common input voltage source and the load. The second current sense signal provides a relatively slow but accurate indication of output current that is not affected by thermal variation. The second current sense signal is filtered to remove noise therefrom. An integrated difference is derived between the first current sense signal and the filtered second current sense signal. Then, the first current sense signal is corrected based on the integrated difference. As in the previous embodiment, the second current sense signal may be derived from the sensing resistor disposed in series between the common input voltage source and a high-side conduction switch of at least one of the plurality of converter modules, or from the sensing resistor disposed in series between the output inductor of the at least one of the plurality of converter modules and the load.
A more complete understanding of the method and apparatus for accurately sensing output current in a DC-to-DC voltage converter will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings that will first be described briefly.